This invention relates to graphene nanoplatelet composites with electroactive materials.
Lithium-ion batteries (LIB) are being pursued for a variety of applications including electric or hybrid electric vehicles (EV & HEV), consumer electronics, grid energy storage systems and distributed power generation units. The success of LIB in such markets strongly depends on the cost, energy density, power capability, cycle life, and safety of the battery cells, which are largely dominated by the electrode materials used. While LIB technologies have advanced significantly since their first commercialization in the early 90's, they are not keeping pace with the ever increasing demand for batteries with higher energy storage capacities. For example, DOE EV Everywhere program defines an EV cell target of 400 Wh/kg with 1000 cycles, at cost of ownership comparable to conventional ICE vehicles. This target cannot be met with current LIB chemistries, especially with graphite as the anode. Because of graphite's low capacity, a cell with a carbon-based anode can reach at most 200-250 Wh/kg, depending on the type of cathode. Therefore, there is a great need to develop advanced anodes for future generation LIB.
Silicon (Si) is considered as the most promising anode material due to its high capacity and proper working voltage. Theoretically, Si can provide up to 4200 mAh/g of Lithium (Li) storage capacity. It can be lithiated in the potential range of 0.0˜0.4 V, which provides the capability to make a high energy density device when paired with an appropriate cathode. Nevertheless, replacing traditional graphite anode materials with Si has proven to be very challenging. Two major barriers have hindered the development of Si-based anodes for commercial applications, especially for use in EV batteries:                (i) Poor cycle life. Si tends to pulverize during cycling as a result of substantial volume change (up to 400%) during charging/discharging of the batteries, which in turn leads to the loss of electrical contact or even disintegration of both Si particles and the electrode coating.        (ii) High synthesis cost. Most of the processes used for the synthesis of Si-based anodes utilize expensive chemical precursors, exotic synthesis methods, or capital-intensive processes. Furthermore, they are usually not suitable for high-volume production. As a result, none of these processes has been successfully commercialized.        
In order to solve the cycling stability problems, researchers have taken various approaches including (a) using nano particles, nanotubes, nano-spheres, and nanowires, (b) applying carbon coating by various methods, or (c) designing porous Si structures. While the capacity, rate capability, and cycling stability have been improved to a certain extent with these processes, the materials usually have relatively low first cycle reversibility and in general still cannot meet the life requirements for most commercial applications.
Silicon particles can be coated by carbon layers via chemical vapor deposition and carbonization processes using carbon precursors such as pitch, glucose, sugar, polyacrylonitrile, polyvinyl alcohol etc. In such coating processes, a carbon thin film 110 on Si 120 surface forms a continuous phase, as illustrated in FIG. 1A, which results in retarding electrolyte penetration and thus 1st cycle efficiency is generally low.
Other approaches involve the coating of silicon particles 140 with graphene materials 130 such as graphenes reduced from graphite oxide and exfoliated graphene, as illustrated in FIG. 1B. In this case, the graphene size is larger than the Si size and one graphene particle makes contact with more than one Si particles. This results in a somewhat rigid framework that cannot easily accommodate the significant volume change during cycling.
The high cost may still be a major obstacle preventing the use of Si as a commercial anode even if the performance is improved. Therefore, a high-performance and low-cost Si-based anode remains a lofty goal of the battery industry.
Further, it has been difficult to build stable full cells with Si-based anodes fabricated with Si nanoparticles, Si nanowire, Si nanotube, Si-alloy, Si/Carbon, and Si/Graphene composite mainly due to continuous growth of a SEI layer and side-reactions between Si and electrolytes in lithiation/delithiation process over extended cycles.
Use of SEI modifier additives to the silicon graphene nanographitic anodes, such as, but not limited to LiF, have been demonstrated as helping improve full cell stability and can be beneficial for stable cell performance.